1. Field of the Invention
The present invention relates to an optical apparatus for reading and/or writing information on a recording medium, such as an optical disk, by means of a light signal, and, more particularly, to an optical head of a fixed type in which the optical apparatus itself does not move between different radial positions of the disk, but only a laser beam moves.
2. Description of the Prior Art
Methods by which information is recorded on and reproduced from recording media are classified roughly into magnetic recording methods and optical recording methods.
In a magnetic recording method, the magnetization direction of magnetic material positioned at the surface of a disk or other substrate is changed electromagnetically, whereby information is recorded and can be reproduced. The recording density in such magnetic recording method is only about 100,000 bits/cm.sup.2.
In contrast, the recording density in present optical recording methods is high, e.g., one million bits/cm.sup.2. Thus, an optical recording method has an advantage compared with a magnetic recording method in permitting recording and reproduction of a large amount of information.
For optical recording and reproducing of information, a conventional optical head has been used which is illustrated in FIG. 1. The conventional optical head comprises a semiconductor laser 1, a partial reflecting mirror 2 through which light passes from the semiconductor laser 1, a condensing lens 9 by which light rays from the partial reflecting mirror 2 are maintained parallel, an objective lens 5 by which light from the condensing lens 9 collects and is radiated to the search portion on the disk 4, a cylindrical lens 6 by which light reflecting from said partial reflecting mirror 2 collects, and an optical detector 7 on which light passing through said lens 6 is detected.
The laser beam which radiates from the semiconductor laser 1 permeates the partial reflecting mirror 2 along the optical axis becomes parallel light rays by passing through the condensing lens 9 and objective lens and forms a focus at the search portion 4a, 4b on the disk.
The laser beam which reflects from such focusing portion reflects again in a predetermined direction from the partial reflecting mirror 2 after passing the objective lens 5 and the condensing lens 3. Then, the laser beam arrives at the optical detector 7 through the cylindrical lens 6 and an electrical current is produced. Therefore, reproduction in accordance with the condition of the focusing portion as it illuminates respective sites on the disk is carried out.
A conventional astigmatism method which is used in detecting the focusing error is explained in relation to FIG. 2. In the case that the shape of laser beam which forms in the optical detector 7 does not have a prescribed shape, an error is produced.
The error is detected as a focusing error of an electric signal. For example, as it is illustrated in FIGS. 2a-2c, an optical detector 7, which is divided into four parts along the diagonal lines, is installed between the first focus (a focus of Z-axis' direction in case the cylindrical lens 6 is not present) and the second focus (a focus of X-axis' direction in case the cylindrical lens 6 is present).
If a focus forms accurately on the disk 4, then the focus of an intersection has the aspect which is illustrated in FIG. 2b. In case the distance between the disk 4 and the objective lens 5 is shorter than the focusing distance, the focusing aspect forms a horizontal oval as it is illustrated in FIG. 2a.
If the distance between the disk 4 and the objective lens 5 is longer than the focusing distance, the focusing aspect forms a vertical oval as it is illustrated in FIG. 2c.
In order that the electric signals of (A+C)-(B+D) which are described in FIG. 2a and FIG. 2b make the aspect which is illustrated in FIG. 2b, the focusing error is corrected by controlling the distance between the objective lens 5 and the disk 4.
A push-pull method which is generally used for tracking is explained in accordance with FIGS. 3a-3c. In case the focus of the laser beam radiates to the tracking groove 4c of the disk 4, the distribution of light intensity forms the aspect which is illustrated in FIG. 3a and FIG. 3c according to the focus inclines toward the right or the left of the tracking groove.
Accurate tracking provides the aspect which is illustrated in FIG. 3b and the electric signals B-D from the optical detector 7 are detected as a tracking error and are corrected.
Signals for error correction are respectively calculated in a servo-circuit and are applied to the focusing coil 8 and the tracking coil 9. Then, the focusing coil 8 and the tracking coil 3 move the objective lens 3 in a direction in which the focusing and tracking error decreases, and continuous adjustments of focus and track are carried out.
In such conventional optical heads, one unit is composed of various optical parts. The various optical parts are actuated by respective motors and are moved to seek positions on the disk for recording, reproducing and searching for information. In accordance with the physical law that the time necessary for accomplishing movement is proportional to the square root of the weight of the object being moved, the time for following a track becomes very long. (For example, the time for following a track is about 100 msec in case of a standard 5.25-inch diameter disk.
Such a long time for following a track is a major factor which hampers the usefulness of the information storage and retrieval device. The portion of objective lens 5 and the receiving and giving portion of light have been operated separately in order that the time for following the track can be improved, and the optical head has been lightened.
In the conventional device, the time for following the track comes to have a limitation that is difficult to decrease, due to the weight of moving parts. The required cost for achieving further reductions in seek time increases because special apparatus has to be prepared in order that the optical head can be moved fast.